Process of carburizing and carburizing composition



Patented Apr. 26, 1932 UNITED STATES PATENT OFFICE GARLETON B. EDWARDS, or INDIANAPOLIS, INDIANA, ASSIGNOR TO PETER c. REILLY,

I or INDIANAPOLIS, INDIANA PROCESS OF cARBURIzlNG AND CARBURIZING COMPOSITION No Drawing. Application filed April 29, 1927, Serial No. 187,706. Renewed December 9, 1930.

carburized by the hot dump process at temperatures varying between 1550 to 1700 F.

(843 to 927 C.). It has become the standard practice to quench certain carburized articles directly from the furnace at the carburizing temperature. For example, in box packing, the contents of the boxes are dumped onto ascreen, the steel articles passing Into a quenching medium, and the carburizing composition dropping through the A screen into cars or onto the floor, where it is allowed to burn or is subjected to a water bath. It is general rotary tube furnace carburizing practice to directly quench in the manner referred to.

In the operation of the direct quench carburizing process, as set forth, it has been customary to use a carburizing composition which burns or ashes. In some plants it is quite common for the hot carburizing material, after use in the carburizing step, to burn and ash for several days. When this is allowed, before the carburizing material can be reused, it-is often necessary to add fresh material, averaging in many cases as high as As an alternative procedure, instead of allowing the carburizer to burn and ash, it is subjected to a water quenching but this is unsatisfactory dueto the washing away of the soluble activators present in the carburizer. Further, the carburizer must be dried before it is satisfactory for reuse.

The carburizer burns and ashes for several days due to its inherent characteristic of combusting rapidly on being dumped red hot from the carburizing pots or furnaces. This burning and ashing results in a notable wastage of the carburizing material The rapid combustion of the carburizing material or composition is due, in a great measure, to its high porosity and its low ignition temperature. Further, the rapid combustion is influenced by the ash and volatile content of the carburizing material.

The common carburizing compositions are, in general, composed of such materials as wood charcoal, gas coke or petroleum coke containing chemical energizer-s bound on to the particles with molasses or equivalent binders, or soaked into the particles. and energizers may also be made into pellets and heated to coke the coal. Charcoal products are especially characterized by a rapid combustion. The carburizing composition made by coking coal and an activator also possesses rapid combusting qualities, due largely to thehigh porosity of the coked mass, its low ignition temperature, the oxygen always present in coal, and the large amount of coal ash and volatile matter. The high porosity is probably due principally to the rapid coking of the coal in the presence of a relatively large quantity of an activator and ash functioning to separate the carbon particles, and to present an enormous surface area favoring rapid combustion when the j used carburizing material is dumped hot from the boxes or pots. j

The present invention has for one of its objects the case-hardening or carburizing of iron and steel articles by a hot process in the presence of a carburizing composition characterized by the property of becoming extinguished without appreciable burning or ashing when it is poured, after use in the carburizing step, red hot from the carburizing boxes or furnaces.

It is a further object of this invention to produce a carburizing composition that is substantially free from porosity, so low in combustibility that it cools below its ignition temperature readily in air and, at the same time has the following qualities which are needed for the best results and greatest economies in steel carburization, namely, low volumetric and weight loss on first use, low ash, sulfur and volatile content, high ignition temperature, comparatively low weight per cubic foot contrasting notably with some of the carburizing compositions now used, high strength of particle, and low specific heat.

Having conceived the desirability of hot carburizing in the presence of a substantially non-burning carburizing material, it follows Coalthat the next step is to produce this material. The solution of this problem involves as one of its elements the selection of the raw carbonaceous material. One of the best raw materials is coal tar carbon produced in accordance with the disclosurein U. S. Patent No. 1,230,782, to Reilly. This material is low in ash and sulfur content. However, any other carbon base may be used which is sufficiently a low porosity, a relatively high ignition temperature and is substantially non-burning in air at the temperatureused in hot carburizing. For example, certain petroleum cokes may be used'as the carbonaceous base, providing their properties, including freedom from ash and sulfur, come-within the specifications referred to. It is desired to indicate that while the preferred material is the coal tar carbon referred to, broadly the invention contemplates the use of any carbonaceous base having the desired properties which have. been briefly referred to,.and willbe later more specifically indicated;

Specifically, a suitable carbonaceous-base is one having. a sulfur content between 10% and and. an ash content between .15% and 2.0%, the latter, of course, being exclusive of j the ash content of any added energizers such burning product. In order to produce a nonburning product, it is necessary to carbonize slowly with but little inorganic material present and bake at a temperature between 600 and 850 C. It is of real importance to have little inorganic matter present during the carbonizing step' as small amounts of some inorganic products act as catalytic agents functioning tolower the ignition temperature and increase the combustibility. of the finished carburizi'ng material, which, of course, is to be avoided W'hilein general the. latter is true, I have found that some. substances, for

,example sodium carbonate, increase the strength'andhardness of the carbonized mass,

" which isdesirable in rotarvcarburizing work.

Further, the purity ofthecarbon base including its freedom-fromash, not only causes lower combustibility, but also gives a longer life to the carburizing composition, since there is .no ash to neutrahze activators by forming silicates and other compounds.

In general, the carbonizmg process should I be carried out slowly and at the correct temperature sothat most of the volatile matter will be driven off and simultaneously there pure and which, when thoroughly baked, has.

following mixes, although, it is, of'course, un- 1 derstood that the same may be varied.

Per cent Per cent Reilly carbon 60 50 Sodiumcarbonate 1.5 4 Barium caroonate 7.5 12 Calcium carbonate 1.5 6

The carbon in either of the above mixes is in the form of small sized particles preferably between two and twenty mesh. The mix is made into carburizing particles to which is added a hard pitch melting at a suitable temperature, preferably around 200 C. Pitches may be used having melting or softening pjoints varying between 140 C. and 325 C.

oal tar pitch is the most suitable coating agent. However, petroleum pitch, wood tar pitch and vegetable oil pitch may be used if they have a low ash and sulfur content. In

the above examples the addition of 29.5%

' being close to the latter to produce a good hard particle. It is important that thepassage of the mix through the kiln be regulated so that the baking operation is not performed too quickly. Using a kiln about twenty to thirty feet long, the time of passage should approximately be between 10 and 15 minutes. The baking is to be carried out so as to produce a carburizing composition having a volatile content between 2% and 121%, and of low specific heat.

The carburizing compound may also be made by incorporating the energizer in coal tar or coal tar pitch and coking the mass, or by causing the energizers to'adhere to the carbon by binders, any of these methods being so carried out that the porosity of the material is so low, and the ignition temperature so high, that the material ceases to burn materially when dumped hot.

The product resulting from the proper choice of raw materials and a regulated baking action may be termed a non-burning material since it does not burn appreciably upon exposure to air after use in the carburizing step, does not ash enough to be visible under ordinary conditions of use, and therefore, for all practical purposes, is non-burning.

The carburizingmaterial resulting from baking out process.

ing, for example, between 0.3% and 1.8%.

In general, it may be stated that the burning loss on hot dumping is usually under 0.5% under conditions where using the usual car- 'burizing material, there is a loss averaging about 10% but which may vary between 5% and 38%.

When using the substantially non-burning carburizing material, it is found that the addition of 5% to 20% of new material keeps up the volume and carburizing activity, even under hot dumping conditions. Due to the method of baking employed including the employment of a relatively high temperature, the original volatile loss on a first run carburizing operation is from 2% to 12% instead of 15% to 40% when using carburizing compositions not produced in accordance with the method set forth.

Utilizing the non-burning carburizing material, the case-hardening or carburizing of steel articles by the direct-quench process may be carried out with an enormous saving of the case-hardening or carburizing composition. Further, the process may be carried out without quenching the carburizing. -material after use, thereby permitting the economical employment of soluble activators in the carburizer. Ordinarily, if these are present in the burning type of carburizing material, they are'lost, when the material is subjected to a water-quenching to extinguish the carburizer.

The differences existing between the nonburning type of carburizing composition and the charcoal-coke products and coal and activator coked masses, are:

1. Upon dumping in the air at red heat, the non-burning product becomes extinguished without burningwhereas the others continue to glow, as does charcoal, and may ash completely. The following data are illustrative Non-burning type product shows 0.4% to 1.8% burning loss; others under similar conditions 5.2% to 37.7%.

2. The non-burning carburizing composition is black or grey, carbon color-after direct quench. Other products show the white ash resulting from substantially complete or partial combustion.

3. In order to show the marked difference between the non-burning composition and other products, place the former and either a charcoal compound or coal pellets side by side, on a wire screen. Then place a burner below and heat to red heat. Remove the flame The non-burning product darkens,

ceases burning rapidly, and shows no ash. The other continues to glow, shows white ash and in many cases ashes completely.

4. At a given temperature, for example, 500 C. the non-burning product burns much more slowly (does not glow). Another way of stating this is, it requires a higher temperature to give the same degree of glowing or burning.

5. In one series of tests, equal quantities of several materials were heated to a carburizing temperature (1650 F.) and dumped hot. The burningloss on the non-burning prodnot was 0.4% to 11% as against a range for the others of from 9.5% to 37.7%. Under different conditions of test these values will be different, but of the same relative order.

These are merely indicative of the difference between the non-burning product and others, showing more clearly how and why such inl provement and economies are shown by nonburning material in actual plant usage.

6. Another experiment showing similar results was carried out as follows: Carburizing particles of various kindswere heated in a flame to between 800 and 900 (1.. The flame was then removed and the particles blown by the breath or a stream of air. All of the compositions glowed brightly with the exception of the non-burning material which was quickly extinguished by the air current.

7. Actual ignition temperatures were determined by placing several particles of a carburizing composition in a glass tube in an electric furnace, carrying the furnace tube and composition to a given temperature and passing a stream of air over the material.

The ignition point was taken as the lowest temperature at which the particles would glow when the air was passed over them.

The non-burning composition first showed glowing at 475 C. to 550 0. (different samples). The others glowed between 325 and 415 C. under similar conditions. It therefore appears that a temperature of 450 C.

is .well above the ignition point of any carburizing composition heretofore used, whereas the same temperature is below the ignition point of the non-burning composition. The non-burning carburizing composition is characterized by an ignition temperature of above 450 and this characteristic distinguishes the new product from compositions heretofore.

stream of air' was continued. In the case. of all compounds other than the non-burning type, the particles continued to glow and ash after they were removed from the external source of heat. The non-burning type, however, even though glowingwhen removed, rapidly dropped in temperature and became'black almost at once. The difference here, of course, is due to the lower rate of burning in the case of the non-burning type.

Its combustion under these conditions is 111- sufiicient to keep the particle up to orabove its ignition point. The combustibility being lowerthan other carburizing compounds and the ignition temperature being higher, both contribute to the production of a carburizing composition admirably suited for carburizing economically by a hot process.

' In practically all hot dump procedures the steel and carburizing material are emptied from the furnace or pots at from1550 to 1800 F. (843 to'982 which is considerably above the ignition point of any of the carburizing compositions. Therefore, when a carburizing composition is poured into the air at this temperature, ease of burning, the porosity of the composition and the ignition temperature in air, are all contending factors causing the composition to be substantially non-burning. While porosity is a very important factor, it is not the only cause of a high burningrate and long continued burning in air. Small amounts of certain catalytic agents will function to lower the ignition temperatureand increase the combustibility of carbonaceous materials. In general, it may be stated that in a. non-burning carburizing composition, the presence of impure carbon, porous carbon, easily burning carbon and oxides, particularly oxides of lead, should be avoided. This may be expressed a little differently by stating that the raw materials should be so selected and the process of treatment so carried out, as has been indicated in illustrated examples, to produce an allotropic modification of carbon which does not burn.

Summarizing, it may be stated that the new product is characterized by the following properties:

1. Itsvolatile content varies between 2% and 12%instead of 15% and 40% as in products heretofore used. This lower weight loss results in great economy.

2. It possesses highheat conductivity.

. 3. The ash content exclusive of carbonate is between 1.5% and 2.0%. In compositions heretofore used, the ash content has varied between 5% and 10%. Low ash means little fusion of the activators with the ash thereby preventing loss in carburizing efficiency.

4. The sulfur content varies between .10% and .40-%.

5. It has a low specific heat (approximately 3%). Being baked more thoroughly and 86 pounds per cubic foot, contrasting notably with some of the carburizing compositions now used, which usually weigh approximately'44 pounds per cubic foot.

9. The strength of the non-burning carburizing particles is such as to favorably resist wear and crushing in use. The particle is hard and firm; much more so than charcoal products, which are severely ground in the rotary carburizing furnace.

The non-burning carburizing composition herein set forth may be provided with a noncarburizing shell or coating, preferably of carbon, which serves as an insulator to prevent contact between the materials in the shell and; the article being carburized.

By the term iron articles as used in the specification and claims, it is intended to include articles'made from iron, steel, and iron and steel alloys.

By the term non-burning carburizing composition as used in the specification and claims, it is intended to define a composition which does not ash enough to be visible, and appreciably burn upon exposure to air after use in a carburizing operation carried out at a relatively high temperature.

I claim:

1. As a new article of manufacture a sub stantially non-burning carburizing composition having as an essential element a predominating quantity of substantially nonporous carbon.

2. As a new article of manufacture a substantially non-burning carburizing composition consisting mainly of a substantially nonporous carbon of low combustibility and having a volatile content averaging between 2% and 12%.

3. As a new article of manufacture a substantially non-burning carburizing composi-- tion having as an essential element a: predominating quantity of substantially nonporous carbon and likewise containing a chemical energizer.

5. As a new article of manufacture, a sub- 7 stantially non-burning carburizing composi- 

